I always thought the flood were an ever-mutating parasite that survived by mutating to adapt to different galaxies.
And they travelled the universe in search of different life forms which they would kill and leech off the corpses to alter their DNA and mutate more, eventually they'd have killed everything in the universe and would die because there would be nothing left to leech off of, since, after all, I thought they were A BIG, GENETICLY-WRONG, DISGUSTING, UGLY PARASITE! :D